U.S. patent number 6,846,718 [Application Number 09/857,803] was granted by the patent office on 2005-01-25 for method for producing soi wafer and soi wafer.
This patent grant is currently assigned to Shin-Etsu Handotai Co., Ltd.. Invention is credited to Hiroji Aga, Susumu Kuwabara, Kiyoshi Mitani, Naoto Tate.
United States Patent |
6,846,718 |
Aga , et al. |
January 25, 2005 |
Method for producing SOI wafer and SOI wafer
Abstract
A method for producing an SOI wafer by the hydrogen ion
delamination method comprising at least a step of bonding a base
wafer and a bond wafer having a micro bubble layer formed by gas
ion implantation and a step of delaminating a wafer having an SOI
layer at the micro bubble layer as a border, wherein, after the
delamination step, the wafer having an SOI layer is subjected to a
two-stage heat treatment in an atmosphere containing hydrogen or
argon utilizing a rapid heating/rapid cooling apparatus (RTA) and a
batch processing type furnace. Preferably, the heat treatment by
the RTA apparatus is performed first. Surface roughness of an SOI
layer surface delaminated by the hydrogen ion delamination method
is improved over the range from short period to long period, and
SOI wafers free from generation of pits due to COPs in SOI layers
are efficiently produced with high throughput.
Inventors: |
Aga; Hiroji (Gunma,
JP), Tate; Naoto (Gunma, JP), Kuwabara;
Susumu (Gunma, JP), Mitani; Kiyoshi (Gunma,
JP) |
Assignee: |
Shin-Etsu Handotai Co., Ltd.
(Tokyo, JP)
|
Family
ID: |
17777984 |
Appl.
No.: |
09/857,803 |
Filed: |
June 11, 2001 |
PCT
Filed: |
October 13, 2000 |
PCT No.: |
PCT/JP00/07111 |
371(c)(1),(2),(4) Date: |
June 11, 2001 |
PCT
Pub. No.: |
WO01/28000 |
PCT
Pub. Date: |
April 19, 2001 |
Foreign Application Priority Data
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|
|
|
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Oct 14, 1999 [JP] |
|
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11-292134 |
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Current U.S.
Class: |
438/406;
257/E21.568; 438/458; 438/506; 438/514; 438/528; 438/977 |
Current CPC
Class: |
H01L
21/76254 (20130101); Y10S 438/977 (20130101) |
Current International
Class: |
H01L
21/762 (20060101); H01L 21/70 (20060101); H01L
021/76 () |
Field of
Search: |
;438/458,406,506,514,528,977,FOR 365/ ;438/FOR 386/ ;438/FOR 403/
;438/FOR 408/ ;438/FOR 158/ ;438/FOR 485/ ;148/DIG.12 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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19753494 |
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Oct 1998 |
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DE |
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A 7-321120 |
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Dec 1995 |
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JP |
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A 8-264552 |
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Oct 1996 |
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JP |
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A 10-242154 |
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Sep 1998 |
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JP |
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A 10-275905 |
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Oct 1998 |
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JP |
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10275905 |
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Oct 1998 |
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JP |
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A 10-335616 |
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Dec 1998 |
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JP |
|
A 11-145436 |
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May 1999 |
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JP |
|
A 11-186277 |
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Jul 1999 |
|
JP |
|
A 11-191617 |
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Jul 1999 |
|
JP |
|
11316154 |
|
Nov 1999 |
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JP |
|
2000012864 |
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Jan 2000 |
|
JP |
|
Other References
Wolf, Stanley, "Silicon Processing for the VLSI Era", vol. 1, 1986
by Lattice Press, pp. 23-25..
|
Primary Examiner: Fourson; George
Assistant Examiner: Estrada; Michelle
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
What is claimed is:
1. A method for producing an SOI wafer by the hydrogen ion
delamination method comprising at least a step of bonding a base
wafer and a bond wafer having a micro bubble layer formed by gas
ion implantation and a step of delaminating a wafer having an SOI
layer at the micro bubble layer as a border, wherein, after the
delamination step, the wafer having an SOI layer is subjected, in
an atmosphere containing hydrogen or argon, to both a heat
treatment utilizing a rapid heating/rapid cooling apparatus to
improve the surface roughness of short periods of the SOI layer and
a heat treatment utilizing a batch processing type furnace to
improve the surface roughness of long periods of the SOI layer.
2. The method for producing an SOI wafer according to claim 1,
wherein the two-stage heat treatment is performed by subjecting the
wafers to a heat treatment in the rapid heating/rapid cooling
apparatus and then a heat treatment in the batch processing type
furnace.
3. A method for producing an SOI wafer by the hydrogen ion
delamination method comprising at least a step of bonding a base
wafer and a bond wafer having a micro bubble layer formed by gas
ion implantation and a step of delaminating a wafer having an SOI
layer at the micro bubble layer as a border, wherein an FZ wafer,
an epitaxial wafer or a CZ wafer of which COPs at least on surface
are reduced is used as the bond wafer, and the wafer having an SOI
layer is subjected to a heat treatment under an atmosphere
containing hydrogen or argon in a batch processing type furnace
after the delamination step.
Description
TECHNICAL FIELD
The present invention relates to a method for producing an SOI
(Silicon On Insulator) wafer, more specifically, a method for
producing an SOI wafer by the so-called hydrogen ion delamination
method (also called Smart Cut Method (registered trademark))
comprising bonding an ion-implanted wafer to another wafer that
serves as a substrate and then delaminating the wafers to produce
an SOI wafer, in which surface roughness is improved by a heat
treatment after the delamination, and an SOI wafer produced by the
method. Further, the present invention also relates to a method for
producing an SOI wafer that can reduce bonding failures during the
production of the SOI wafer and can provide SOI wafers with good
yield, in which a heat treatment is performed after the
delamination.
BACKGROUND ART
Recently, as a method for producing an SOI wafer, the method
comprising bonding a wafer implanted with hydrogen ions or the like
and then delaminating the wafer to produce an SOI wafer (a
technique called hydrogen ion delamination method: Smart Cut Method
(registered trademark)) is newly coming to attract much attention.
This method is a technique for producing an SOI wafer, wherein an
oxide layer is formed on at least one of two silicon wafers, at
least either hydrogen ions or rare gas ions are implanted into one
wafer from its top surface to form a micro bubble layer (enclosed
layer) in this silicon wafer, then the ion-implanted surface of the
wafer is bonded to the other silicon wafer via the oxide layer,
thereafter the wafers were subjected to a heat treatment
(delamination heat treatment) to delaminate one of the wafer as a
thin film at the micro bubble layer as a cleavage plane, and the
other wafer is further subjected to a heat treatment (bonding heat
treatment) to obtain an SOI wafer in which an SOI layer is firmly
bonded on the silicon wafer (refer to Japanese Patent Laid-open
(Kokai) Publication No. 5-211128).
When an SOI wafer is produced by the hydrogen ion delamination
method, the SOI layer surface as it is after the delamination at
the micro bubble layer as a cleavage plane has higher surface
roughness compared with a mirror-polished wafer used for usual
device production, and therefore the wafer as it is cannot be used
for the device production. Accordingly, in order to improve the
aforementioned surface roughness, polishing using a small amount of
stock removal for polishing, which is called touch polish, is
usually performed.
However, the SOI layer is extremely thin, and therefore when its
surface is polished, there is caused a problem that variation in
SOI layer thickness becomes large due to fluctuation of the
polishing amount within the surface.
Therefore, it was proposed to improve the surface roughness by a
heat treatment of the SOI layer surface immediately after the
delamination, without using polishing.
Japanese Patent Laid-open (Kokai) Publication No. 10-242154
discloses a method wherein, after a second heat treatment for
strengthening bonding of a support substrate and a single crystal
silicon thin film (bonding heat treatment), a third heat treatment
is performed at a temperature of 1000-- 1300.degree. C. for 10
minutes to 5 hours in a hydrogen atmosphere to improve average
surface roughness of the silicon thin film.
Further, Japanese Patent Laid-open (Kokai) Publication No.
10-275905 discloses a method for producing an SOI wafer wherein a
wafer of the SOI structure having a delaminated surface, which is
obtained by the hydrogen ion delamination method, is subjected to
annealing in a hydrogen atmosphere (hydrogen annealing) to flatten
the delaminated surface.
Thus, any of the techniques disclosed in the aforementioned patent
documents utilizes a heat treatment in a hydrogen atmosphere to
improve the surface roughness of a delaminated wafer.
The aforementioned Japanese Patent Laid-open (Kokai) Publication
No. 10-242154 defines temperature and time for the third heat
treatment (hydrogen annealing) for improving the average surface
roughness. However, if, for example, the SOI layer (single crystal
silicon thin film) is formed from a wafer produced by the
Czochralski method (CZ method) and it has a small thickness of
about 0.5 .mu.m or less, there is caused a problem that a buried
oxide layer is etched by hydrogen gas penetrated through COP
(Crystal Originated Particle), which is a void-like grown-in
defect, when the hydrogen annealing is performed. Further, although
it is known that a heat treatment performed in an argon atmosphere
also improves the surface roughness like the heat treatment in
hydrogen, however, it also cannot obviate the problem of etching
through COP. That is, it is known that a CZ wafer has crystal
defects called COPs introduced therein during the crystal growth,
and it has become clear that, if such a CZ wafer is utilized for
the bond wafer to be a device active layer, COPs exist also in the
SOI layer and in a case of an extremely thin SOI layer, which is
required in recent years, the COPs penetrate the SOI layer and form
pinholes to markedly degrade electric characteristics.
Meanwhile, Japanese Patent Laid-open (Kokai) Publication No.
10-275905 discloses that, as specific methods for the heat
treatment (annealing), the heat treatment can be performed by any
one of short time annealing (rapid thermal anneal, RTA) of the
single wafer processing in which wafer is treated one by one and
plasma annealing, besides the method of hydrogen annealing
performed for several tens of seconds to several tens of minutes in
a hydrogen atmosphere using a batch processing type furnace.
Among the aforementioned various heat treatments (annealing), the
rapid thermal annealing (RTA) utilizing a rapid heating/rapid
cooling apparatus can be performed within an extremely short period
of time. Therefore, it was considered that the aforementioned
buried oxide layer was not etched, COPs in the SOI layer could be
eliminated simultaneously, and thus the surface roughness could be
improved efficiently.
However, when the inventors of the present invention precisely
investigated the improvement of the surface roughness of SOI wafer
by RTA, it was found that it was only short period components of
surface roughness that were improved to a level comparable to that
of mirror-polished wafers for the usual device production, and long
period components were still extremely inferior to those of the
mirror-polished wafers.
When the relationship between the heat treatment time and the
surface roughness was further investigated, it was found that, in
order to improve the long period components of surface roughness by
an RTA apparatus, a heat treatment of high temperature for long
period of time (for example, at 1225.degree. C. for 3 hours or
more) was required.
However, since the heat treatment performed by an RTA apparatus is
one of the single wafer processing type, treatment for a long
period of time lowers throughput and degrades efficiency. In
addition, it increases the production cost. Therefore, it is not
practical.
On the other hand, although a batch processing furnace that enables
a heat treatment for a long period of time can treat a lot of
wafers at one time, it suffers from a problem that the buried oxide
layer is etched through COPs in the SOI layer during the hydrogen
annealing treatment to form pits due to the slower temperature
increasing rate.
DISCLOSURE OF THE INVENTION
The present invention was accomplished in order to solve the
aforementioned problems, and its object is to improve surface
roughness over the range from short period to long period of an SOI
layer surface delaminated by the hydrogen ion delamination method
without polishing and to secure its thickness uniformity, as well
as to efficiently produce SOI wafers free from generation of pits
due to COPs in SOI layers with high throughput.
In order to achieve the aforementioned object, the present
invention provides a method for producing an SOI wafer by the
hydrogen ion delamination method comprising at least a step of
bonding a base wafer and a bond wafer having a micro bubble layer
formed by gas ion implantation and a step of delaminating a wafer
having an SOI layer at the micro bubble layer as a border, wherein,
after the delamination step, the wafer having an SOI layer is
subjected to a two-stage heat treatment in an atmosphere containing
hydrogen or argon utilizing a rapid heating/rapid cooling apparatus
and a batch processing type furnace.
If a wafer having an SOI layer is subjected to a heat treatment
consisting of two stages utilizing separately a rapid heating/rapid
cooling apparatus and a batch processing type furnace after the
delamination as described above, surface crystallinity is restored
and the surface roughness of short periods is improved in the heat
treatment by the rapid heating/rapid cooling apparatus, and the
surface roughness of long periods can be improved by the heat
treatment utilizing the batch processing type furnace. Further,
since a plurality of wafers can be subjected to a heat treatment at
one time in the batch processing type furnace, the wafers can be
produced with higher throughput compared with a case where wafers
are subjected to a heat treatment for a long period of time by the
single wafer processing in a rapid heating/rapid cooling
apparatus.
Furthermore, since this method does not use polishing such as touch
polishing, thickness uniformity of the SOI layer is also
secured.
Further, in the aforementioned method, the two-stage heat treatment
is preferably performed by subjecting the wafers to a heat
treatment in the rapid heating/rapid cooling apparatus and then a
heat treatment in the batch processing type furnace.
In the present invention, both of the short period components and
long period components of surface roughness are improved by the
two-stage heat treatment as described above. If the heat treatment
in the rapid heating/rapid cooling apparatus for a short period of
time is performed as the first stage, crystallinity of the surface
is restored and COPs in the SOI layer are markedly reduced.
Therefore, when the heat treatment by the batch processing type
furnace is performed in a subsequent stage, COPs in the SOI layer
are substantially eliminated already. Accordingly, even if the heat
treatment is performed for a relatively long period of time, the
etching of the buried oxide layer by hydrogen gas or argon gas,
which is caused through penetrated COPs, is suppressed, and thus
pits are not generated.
Further, the present invention also provides a method for producing
an SOI wafer by the hydrogen ion delamination method comprising at
least a step of bonding a base wafer and a bond wafer having a
micro bubble layer formed by gas ion implantation and a step of
delaminating a wafer having an SOI layer at the micro bubble layer
as a border, wherein an FZ wafer, an epitaxial wafer or a CZ wafer
of which COPs at least on surface are reduced is used as the bond
wafer, and the wafer having an SOI layer is subjected to a heat
treatment under an atmosphere containing hydrogen or argon in a
batch processing type furnace after the delamination step.
If an SOI wafer is produced by using any one of an FZ wafer, an
epitaxial wafer and a CZ wafer of which COPs at least on surface
are reduced is used as the bond wafer as described above, COPs in
the SOI layer can be reduced or substantially completely
eliminated. Therefore, the problem of etching of the buried oxide
layer due to COPs is not caused, and a heat treatment at a high
temperature for a long period of time in a batch processing type
furnace also becomes possible.
Further, another object of the present invention is to provide SOI
wafers with good yield by reducing bonding failures such as voids
and blisters generated at a bonding surface, when SOI wafers of
which COPs in the SOI layer are reduced are produced as described
above.
To this end, the present invention also provides a method
characterized by using a CZ wafer produced from a single crystal
ingot of which COPs are reduced for the whole crystal as a wafer
used for the bond wafer.
If a CZ wafer produced from a single crystal ingot of which COPs
are reduced for the whole crystal is used as described above, a
usual mirror-polished surface can be used as a bonding surface, and
therefore bonding failures can be reduced compared with a case
utilizing an epitaxial wafer. Further, since the method utilizes a
CZ wafer, it can be applied to a wafer having a large diameter such
as 200 mm, 300 mm or a further larger diameter, which are
considered difficult to be produced for FZ wafers. Furthermore,
since COPs are reduced for the whole crystal (whole wafer), the
stock removal of the delaminated plane for polishing is not
required to be limited, when the delaminated wafer is recycled as a
bond wafer.
Further, by subjecting an SOI wafer produced as described above, in
which COPs in the SOI layer are reduced, to a heat treatment under
an atmosphere containing hydrogen or argon in a batch processing
type furnace, the surface roughness of the SOI layer can be reduced
without producing pits of the buried oxide layer.
According to the present invention, there is further provided an
SOI wafer which is produced by the aforementioned method,
characterized in that the wafer has an RMS (root mean square
roughness) value of 0.5 nm or less concerning surface roughness for
both of 1 .mu.m square and 10 .mu.m square.
Thus, in the SOI wafer produced according to the present invention,
although it is produced without polishing, both of the short period
components (for example, about 1 .mu.m square) and the long period
components (for example, about 10 .mu.m square) of the surface
roughness of the SOI layer are improved, and both of RMS values
therefor are very small, i.e., 0.5 nm or less, which means surface
roughness comparable to that of mirror-polished wafers. In
addition, the film thickness does not become uneven unlike in a
case where the surface is polished. Therefore, such an SOI wafer
can suitably be used for the production of recent highly integrated
devices.
As explained above, in the method for producing an SOI wafer of the
present invention, by subjecting a wafer having an SOI layer to a
two-stage heat treatment utilizing a rapid heating/rapid cooling
apparatus and a batch processing type furnace in an atmosphere
containing hydrogen or argon after the delamination step, both of
short period components and long period components of surface
roughness of delaminated plane of the wafer can be markedly
improved. Further, crystallinity is also restored, and pits due to
COPs in the bond wafer to be used are not generated.
Furthermore, the short period components of surface roughness are
improved within an extremely short period of time by the heat
treatment using an RTA apparatus, and in addition, a lot of wafers
can be processed at one time and the long period components are
improved in a batch processing type furnace. Therefore, the heat
treatment can be efficiently performed as a whole, and thus SOI
wafers of superior surface characteristics can be produced at a low
cost.
BRIEF EXPLANATION OF THE DRAWINGS
FIG. 1(a) to (h) show a flow diagram of an exemplary process for
producing an SOI wafer by the hydrogen ion delamination method
according to the present invention.
FIG. 2 shows a graph representing relationship among RTA treatment
temperature, treatment time, and P-V value for 1 .mu.m square.
FIG. 3 shows a graph representing relationship among RTA treatment
temperature, treatment time, and P-V value for 10 .mu.m square.
FIG. 4 shows a graph representing relationship among RTA treatment
temperature, treatment time, and RMS value for 1 .mu.m square.
FIG. 5 shows a graph representing relationship among RTA treatment
temperature, treatment time, and RMS value for 10 .mu.m square.
FIG. 6 is a schematic view showing an exemplary rapid heating/rapid
cooling apparatus.
FIG. 7 is a schematic view showing another exemplary rapid
heating/rapid cooling apparatus.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereafter, embodiments of the present invention will be explained
with reference to the appended drawings. However, the present
invention is not limited to these.
FIG. 1 shows a flow diagram of an exemplary process for producing
an SOI wafer by the hydrogen ion delamination method according to
the present invention.
The present invention will be explained hereafter mainly as for a
case where two of silicon wafers are bonded.
In the hydrogen ion delamination method shown in FIG. 1, two
mirror-surface silicon wafers are prepared first in the step (a).
That is, a base wafer 1 that serves as a substrate and a bond wafer
2 from which an SOI layer is obtained, which correspond to
specifications of devices, are prepared.
Then, in the step (b), at least one of the wafers, the bond wafer 2
in this case, is subjected to thermal oxidation to form an oxide
layer 3 having a thickness of about 0.1-2.0 .mu.m on its
surface.
In the step (c), at least either hydrogen ions or rare gas ions,
hydrogen ions in this case, are implanted into one surface of the
bond wafer 2 on which surface the oxide layer was formed to form a
micro bubble layer (enclosed layer) 4 parallel to the surface in
mean penetrating depth of the ions. The ion implantation
temperature is preferably 25-450.degree. C.
The step (d) is a step of superimposing the base wafer 1 on the
hydrogen ion-implanted surface of the hydrogen ion implanted bond
wafer 2 via an oxide layer and bonding them. By contacting the
surfaces of two of the wafers to each other in a clean atmosphere
at an ordinary temperature, the wafers are adhered to each other
without using an adhesive or the like.
The subsequent step (e) is a delamination heat treatment step in
which the wafers were delaminated at the enclosed layer 4 as a
border to separate them into a delaminated wafer 5 and a wafer 6
having an SOI layer (SOI layer 7+buried oxide layer 3+base wafer
1). For example, if the wafers are subjected to a heat treatment at
a temperature of about 500.degree. C. or more under an inert gas
atmosphere, the wafers are separated into the delaminated wafer 5
and the wafer 6 having an SOI layer due to rearrangement of
crystals and aggregation of bubbles (this wafer may be simply
called SOI wafer hereinafter including such a wafer subjected to a
heat treatment).
As for the steps thus far, the method of the present invention is
the same as the conventional hydrogen ion delamination method. And
in the present invention, the method is characterized by subjecting
the wafer 6 having an SOI layer 7 to a two-stage heat treatment in
an atmosphere containing hydrogen or argon using a rapid
heating/rapid cooling apparatus and a batch processing type furnace
(step (g)), after the delamination heat treatment step (e). In this
case, the atmosphere containing hydrogen or argon may consist of
100% of hydrogen, 100% of argon or a mixed gas of hydrogen and
argon.
In addition, after the delamination heat treatment step (e) and
before performing the two-stage heat treatment step (g), the
bonding heat treatment may be performed in the step (f) like a
conventional method. Since the bonding strength of the wafers
brought into close contact in the aforementioned bonding step (d)
and the delamination heat treatment step (e) as it is would be weak
for use in the device production process, the wafer 6 having an SOI
layer is subjected to a heat treatment at a high temperature as a
bonding heat treatment in this step (f) to obtain sufficient
bonding strength. This heat treatment is preferably performed, for
example, at 1050.degree. C. to 1200.degree. C. for 30 minutes to 2
hours under an inert gas atmosphere.
In the present invention, after the delamination heat treatment
step (e), the wafer 6 having an SOI layer is subjected to the
bonding heat treatment as required, and then subjected to the
two-stage heat treatment in an atmosphere containing hydrogen or
argon using a rapid heating/rapid cooling apparatus and a batch
processing type furnace. In this case, since it is inefficient to
separately perform the bonding heat treatment (step (f)) and the
two-stage heat treatment, the two-stage heat treatment using a
rapid heating/rapid cooling apparatus and a batch processing type
furnace according to the present invention may also serve as the
bonding heat treatment.
As for the order of the heat treatment using a rapid heating/rapid
cooling apparatus and the heat treatment using a batch processing
type furnace, the heat treatment using a rapid heating/rapid
cooling apparatus is preferably performed first, in particular,
when a usual CZ wafer having a lot of COPs is used as a bond
wafer.
This is because of the following reasons. A CZ wafer contains COPs
introduced during the crystal production as described above.
Therefore, when the SOI layer is thin as required in recent years,
the COPs may exist while penetrating the SOI layer to form
pinholes. In such a case, if the wafer is subjected to a heat
treatment in an atmosphere containing hydrogen or argon over a long
period of time in a batch processing type furnace, hydrogen gas or
argon gas may penetrate through the pinholes and etch the buried
oxide layer 3 to form pits during the heat treatment.
Therefore, when a usual CZ wafer is used as the bond wafer, if the
heat treatment by the rapid heating/rapid cooling apparatus is
performed first to improve the short period components of surface
roughness and simultaneously restore the surface crystallinity to
markedly reduce COPs in the SOI layer, and then the heat treatment
is performed in the batch processing type furnace for a relatively
long period of time to improve the long period components, both of
the short period components and long period components of surface
roughness will be improved, and possibility of the generation of
pits will also be eliminated.
On the other hand, if an epitaxial wafer, FZ wafer or CZ wafer of
which COPs at least on surface are reduced is used as the bond
wafer, the aforementioned problem of the etching of the buried
oxide layer is not caused. Therefore, either of the heat treatments
may be performed first in principle, and it also becomes possible
to perform the heat treatment at a high temperature for a long
period of time by using only a batch processing type furnace, while
omitting the heat treatment using a rapid heating/rapid cooling
apparatus.
That is, if the material of the bond wafer is suitably selected, an
SOI wafer excellent in both of the short period components and long
period components of surface roughness can be obtained by
subjecting the wafer to a heat treatment at a temperature of about
1000-1300.degree. C. for 10 minutes to about 5 hours in a batch
processing type furnace, while avoiding the problem of etching of
the buried oxide layer, as in the case of performing the two-stage
heat treatment.
Examples of the CZ wafer of which COPs at least on surface are
reduced include a CZ wafer produced from a single crystal ingot of
which COPs are reduced for the whole crystal by changing a usual CZ
single crystal pulling rate (about 1 mm/min) to a pulling rate of,
for example, 0.6 mm/min or lower, a CZ wafer produced from a single
crystal ingot of which grown-in defects such as COPs are reduced
for the whole crystal by controlling V/G (V: pulling rate, G:
temperature gradient along the direction of solid-liquid interface
of crystal), or a CZ wafer produced with usual pulling conditions
and subjected to a heat treatment in an atmosphere of hydrogen,
argon or the like to reduce COPs contained in at least a region of
wafer surface to be an SOI layer, and so forth.
Through the aforementioned steps (a) to (g), there can be obtained
an SOI wafer 6 of high quality, in which both of the long period
components and short period components of surface roughness are
improved, and which has an SOI layer 7 of high crystal quality and
high thickness uniformity and shows no pit formation (step
(h)).
Further, by selecting a CZ wafer produced from a single crystal
ingot of which COPs are reduced for the whole crystal as the CZ
wafer of which COPs at least on surface are reduced, the following
remarkable advantages can be obtained.
That is, it is considered that, as a bond wafer for producing an
SOI wafer of which COPs in the SOI layer are reduced compared with
an SOI wafer produced by using a CZ wafer produced under the usual
crystal pulling conditions, besides use of a CZ wafer produced from
a single crystal ingot of which COPs are reduced for the whole
crystal, an epitaxial wafer, FZ wafer or CZ wafer subjected to
hydrogen (argon) annealing may be used.
However, in the case of epitaxial wafer, an epitaxial layer is
deposited on a surface of usual mirror-polished wafer, and its
surface roughness (haze level) is degraded compared with a usual
mirror-polished surface. Furthermore, projections called mounds and
the like may be generated on the surface. Therefore, if such a
surface is bonded, bonding failures called voids or blisters are
likely to occur due to the influence of the degraded surface
roughness: or protrusions. Therefore, there may be used a measure
of slightly polishing the epitaxial layer and then using it for
bonding.
On the other hand, in the case of a CZ wafer produced from a single
crystal ingot of which COPs are reduced for the whole crystal,
since a mirror-polished surface of a mirror-surface wafer sliced
from the single crystal and processed can be used as it is, the
bonding failures can be reduced compared with an epitaxial
wafer.
Further, when the bond wafer after the delamination is recycled as
a bond wafer, the delaminated plane must be polished before use.
However, an epitaxial wafer suffers from a problem that, if the
stock removal for polishing becomes large, the epitaxial layer may
be removed. Therefore, it is necessary to take a countermeasure
such as depositing the epitaxial layer with a sufficiently large
thickness beforehand or making the stock removal for polishing
small. This problem is similarly applied to a wafer of which COPs
are reduced only in the vicinity of the wafer surface, like a CZ
wafer subjected to hydrogen (argon) annealing.
In contrast, in the case of a CZ wafer produced from a single
crystal ingot of which COPs are reduced for the whole crystal,
since COPs are reduced for the whole wafer, the stock removal for
polishing is not limited for recycling at all, and the wafer can be
recycled for a plurality of times.
Further, a CZ wafer produced from a single crystal ingot of which
COPs are reduced for the whole crystal is advantageous in view of
the possibility of the production of wafers having a larger
diameter. As for FZ wafers, the maximum diameter of wafers
currently produced on commercial level is 150 mm, and it is
extremely difficult to obtain wafers having a diameter of 200 mm,
300 mm or a further larger diameter. As for CZ wafers, on the other
hand, those having a diameter of 300 mm are already mass-produced,
and study for production of those having a further larger diameter
is also progressing. Therefore, it is well possible to meet the
demand for a larger diameter.
As described above, the production of SOI wafers by the hydrogen
ion delamination method using CZ wafers produced from a single
crystal ingot of which COPs are reduced for the whole crystal as
bond wafers is the only method that simultaneously has three kinds
of advantages, i.e., reduction of bonding failures, reuse of bond
wafers and usability for wafers of a large diameter.
The two-stage heat treatment performed in the present invention
will be explained in more detail hereafter.
First, the heat treatment performed in an atmosphere containing
hydrogen or argon using a rapid heating/rapid cooling apparatus can
be performed in a temperature range of 1000.degree. C. to the
melting point or lower of silicon for 1-300 seconds.
By subjecting a wafer having an SOI layer after the delamination to
a heat treatment in an atmosphere containing hydrogen or argon
using a rapid heating/rapid cooling apparatus, crystallinity of the
SOI layer surface can be efficiently restored in an extremely short
period of time, surface roughness, in particular, short period
components thereof (about 1 .mu.m square) can be improved, and COPs
in the SOI layer can also be markedly reduced. The heat treatment
can be performed more effectively in a temperature range of
1200-1350.degree. C.
Examples of such an apparatus that can rapidly heat and rapidly
cool an SOI wafer in an atmosphere containing hydrogen or argon,
which is used in the present invention, include apparatuses such as
lamp heaters based on heat radiation. As an example of commercially
available apparatuses, SHS-2800 produced by AST Corp. can be
mentioned. These apparatuses are not particularly complicated, and
are not expensive either.
An example of apparatus that can rapidly heat and rapidly cool a
wafer having an SOI layer in an atmosphere containing hydrogen or
argon, which is used in the present invention, will be explained
hereinafter. FIG. 6 is a schematic view of an apparatus capable of
rapid heating and rapid cooling.
The heat treatment apparatus 20 shown in FIG. 6 has a bell jar 21
composed of, for example, silicon carbide or quartz, and a wafer is
heat-treated in this bell jar 21. Heating is performed by heaters
22 and 22', which are disposed so that they should surround the
bell jar 21. These heaters are each constituted by an upper heater
and a lower heater which are separated from each other, so that
electric power supplied to each of them can be independently
controlled. Of course, the heating mechanism is not limited to
this, and the so-called radiant heating, radiofrequency heating and
so forth may also be used. A housing 23 for shielding heat is
disposed outside the heaters 22 and 22'.
A water cooled chamber 24 and a base plate 25 are disposed under a
furnace, and they shut the inside of the bell jar 21 off from the
outer air. An SOI wafer 28 is held on a stage 27, and the stage 27
is fixed at the top of supporting shaft 26, which can be freely
moved upward and downward by a motor 29. The water cooled chamber
24 has a wafer insertion port (not shown in the figure) which can
be opened and closed by a gate valve, so that the wafer can be
loaded into and unloaded from the furnace along the transverse
direction. The base plate 25 is provided with a gas inlet and
exhaust outlet, so that the gaseous atmosphere in the furnace can
be controlled.
By using such a heat treatment apparatus 20 as mentioned above, the
heat treatment of an SOI wafer for rapid heating and rapid cooling
in an atmosphere containing hydrogen or argon is performed as
follows.
First, the inside of the bell jar 21 is heated to a desired
temperature, for example, 1000.degree. C. to the melting point of
silicon, by the heaters 22 and 22', and maintained at that
temperature. By independently controlling the electric power
supplied to each of the separate heaters, temperature profile can
be obtained in the bell jar 21 along its height direction.
Therefore, the heat treatment temperature can be selected by
changing the position of the stage 27, i.e., the length of the
supporting shaft 26 inserted into the furnace. The atmosphere for
the heat treatment is controlled by introducing an atmospheric gas
containing hydrogen or argon through a gas inlet of a base plate
25.
After the inside of the bell jar 21 is maintained at the desired
temperature, an SOI wafer is inserted from the insertion port of
the water cooled chamber 24 by a wafer handling apparatus not shown
in the figure, which is disposed at an adjacent position of the
heat treatment apparatus 20, and placed on the stage 27 waiting at
its lowest position via, for example, a SiC boat etc. At this
point, because the water cooled chamber 24 and the base plate 25
are cooled with water, the wafer is not heated to a high
temperature at that position.
After the SOI wafer is placed on the stage 27, the stage 27 is
immediately elevated to a position of desired temperature of from
1000.degree. C. to the melting point of silicon by inserting the
supporting shaft 26 into the inside of the furnace by the motor 29
so that the SOI wafer on the stage should be subjected to the high
temperature heat treatment. In this operation, because the stage
moves from its lowest position in the water cooled chamber 24 to
the desired temperature position within, for example, only 20
seconds, the SOI wafer will be rapidly heated.
Then, by maintaining the stage 27 at the desired temperature
position for a predetermined period of time (for example, 1-300
seconds), the wafer having an SOI layer can be subjected to the
high temperature heat treatment in an atmosphere containing
hydrogen or argon for the time that the wafer is maintained at the
heating position. When the predetermined time has passed and the
high temperature heat treatment was finished, the stage 27 is
immediately descended by pulling the supporting shaft 26 out from
the furnace by the motor 29, and positioned at the bottom of the
water cooled chamber 24. This descending operation can also be
performed within, for example, about 20 seconds. Because the water
cooled chamber 24 and the base plate 25 are cooled with water, the
wafer having an SOI layer on the stage 27 is cooled rapidly.
Finally, the SOI wafer is unloaded by the wafer handling apparatus
to finish the heat treatment.
When additional SOI wafers are to be heat-treated, those wafers can
be introduced successively into the apparatus and subjected to the
heat treatment, since the temperature in the heat treatment
apparatus 20 is not lowered.
Another example of the rapid heating/rapid cooling apparatus (RTA
apparatus) for SOI wafers used in the present invention will be
explained hereafter.
The heat treatment apparatus 30 shown in FIG. 7 has a chamber 31
consisting of quartz, and a wafer 38 is heat-treated within this
chamber 31. Heating is achieved by heating lamps 32, which are
disposed under and over the chamber and at left and right of the
chamber so that they should surround the chamber 31. Electric power
supplied to these lamps 32 can be independently controlled.
An auto shutter 33 is provided at the gas exhausting side, and it
shuts the inside of the chamber 31 off from the outer air. The auto
shutter 33 has a wafer loading port not shown in the figure, which
can be opened and closed by a gate valve. The auto shutter 33 is
also provided with a gas exhausting outlet, so that the atmosphere
in the furnace can be controlled.
The wafer 38 is placed on a three-point supporting part 35 formed
on a quartz tray 34. A buffer 36 made of quartz is provided at the
gas inlet side of the tray 34, so that it can prevent the wafer 38
from being directly blown by the introduced gas flow.
Further, the chamber 31 is provided with a special window for
temperature measurement, which is not shown in the figure, and the
temperature of the wafer 38 can be measured by a pyrometer 37
installed in the outside of the chamber 31 through the special
window.
By using the heat treatment apparatus 30 mentioned above, the heat
treatment for rapid heating and rapid cooling of a wafer is
performed as follows.
First, the wafer 38 is loaded into the chamber 31 from the loading
port and placed on the tray 34 by a wafer handling apparatus
disposed at an adjacent position of the heat treatment apparatus 30
but not shown in the figure. Then, the auto shutter 33 is
closed.
Subsequently, electric power is supplied to the heating lamps 32 to
heat the wafer 38 to a predetermined temperature, for example,
1100.degree. C. to 1300.degree. C. In this operation, it takes, for
example, about 20 seconds to attain the desired temperature. Then,
the wafer 38 is maintained at the temperature for a predetermined
period of time, and thus the wafer 38 can be subjected to a high
temperature heat treatment. When the predetermined time has passed
and the high temperature heat treatment is finished, output of the
lamps is reduced to lower the temperature of the wafer. This
temperature decrease can be also performed within, for example,
about 20 seconds. Finally, the wafer 38 is unloaded by the wafer
handling apparatus to finish the heat treatment.
As explained above, the heat treatment by using a rapid
heating/rapid cooling apparatus (RTA apparatus) according to the
present invention include a method utilizing such an apparatus as
shown in FIG. 6, wherein a wafer is immediately loaded into a heat
treatment furnace set at a temperature within the aforementioned
temperature range, and the wafer is immediately unloaded after the
aforementioned heat treatment time has passed, a method utilizing
such an apparatus as shown in FIG. 7, wherein a wafer is placed at
a predetermined position in a heat treatment furnace and
immediately heated by lamp heaters or the like, and so forth. The
expressions of "to immediately load" and "to immediately unload"
mean that there are not employed the conventional temperature
increasing and decreasing operations performed over a certain
period of time and the so-called loading and unloading operations
in which wafers are slowly loaded into and unloaded from a heat
treatment furnace. Of course, however, it takes a certain short
period of time to transport the wafer to the predetermined position
in the furnace, and it takes several seconds to several minutes
depending on the performance of a transportation apparatus for
loading a wafer.
When the heat treatment is performed by using such a heat treatment
apparatus as shown in FIG. 6 or 7, the atmosphere for the heat
treatment in an atmosphere containing hydrogen or argon according
to the present invention may be, for example, 100% hydrogen
atmosphere, 100% argon atmosphere or a mixed gas atmosphere of
hydrogen and argon.
If such a heat treatment atmosphere is used, crystallinity of a
damaged layer of SOI wafer surface is surely restored and surface
roughness, in particular, short period components thereof, can be
improved without forming a harmful coated film on the SOI wafer
surface.
Among the heat treatments constituting the two-stage heat treatment
performed in the present invention, the heat treatment performed in
an atmosphere containing hydrogen or argon by using a batch
processing type furnace will be explained hereafter.
The term "batch processing type furnace" used herein means a
so-called batch type heat treatment furnace of, usually, vertical
type or horizontal type, in which a plurality of wafers are placed,
hydrogen gas is introduced, temperature is relatively slowly
elevated to subject the wafers to a heat treatment at a
predetermined temperature for predetermined time, and the
temperature is relatively slowly lowered. Such an apparatus is
capable of heat treatment of a large number of wafers at one time.
Such an apparatus is also excellent in the controllability of
temperature, and hence enables stable operation.
The heat treatment conditions for the batch processing type furnace
are basically the same as those for the aforementioned RTA
apparatus except that the heat treatment time becomes longer. It
can be performed in 100% hydrogen atmosphere, 100% argon atmosphere
or a mixed gas atmosphere of hydrogen and argon at a temperature of
from 1000.degree. C. to the melting point of silicon, and in
particular, it can be performed more effectively at a temperature
range of 1200-1350.degree. C.
By performing a heat treatment in an atmosphere containing hydrogen
or argon using a batch processing type furnace as described above,
the long period components (for example, about 10 .mu.m square) of
surface roughness of SOI wafer can be improved. In particular, if
the heat treatment using the aforementioned batch processing type
furnaces is performed after the heat treatment using the
aforementioned rapid heating/rapid cooling apparatus, surface
roughness of SOI wafers can be improved over the range from short
period to long period, and SOI wafers free from pits generated due
to COPs can be obtained, even if CZ wafers are used as bond
wafers.
Further, compared with the treatment method by using only the rapid
heating/rapid cooling apparatus for a long period of time, the heat
treatment can be performed more efficiently, and SOI wafers
excellent in the surface characteristics can be produced with a
high throughput at a low cost.
The SOI wafer of the present invention produced as described above
can be an SOI wafer of which both of RMS values for 1 .mu.m square
and 10 .mu.m square concerning surface roughness are 0.5 nm or
less.
The SOI wafer of the present invention, of which both of RMS values
for 1 .mu.m square and 10 .mu.m square concerning surface roughness
are 0.5 nm or less as described above, has surface roughness
substantially comparative to that of mirror-polished wafers over
the range from short period to long period and is excellent in the
film thickness uniformity. Therefore, it can be preferably used for
the production of recent highly integrated devices.
The present invention will be specifically explained with reference
to heat treatment tests according to the present invention as well
as examples and comparative examples. However, the present
invention is not limited to these.
<Heat treatment test using RTA apparatus>
Production of SOI wafers:
First, using a base wafer 1 and a bond wafer 2, both of which were
mirror surface silicon wafers produced by the CZ method and having
a diameter of 150 mm, a wafer 6 having an SOI layer was obtained
through delamination of the bond wafer 2 according to the steps (a)
to (e) shown in FIG. 1. In this production, thickness of the SOI
layer 7 was made to be 0.4 .mu.m, and the other major conditions
including those for the ion implantation were as follows.
1) Thickness of buried oxide layer: 400 nm (0.4 .mu.m)
2) Hydrogen implantation conditions: H.sup.+ ions, implantation
energy: 100 keV, implantation dose: 8 .times.10.sup.16
/cm.sup.2
3) Delamination heat treatment conditions: in N.sub.2 gas
atmosphere, 500 .degree. C., 30 minutes
In this way, the wafer 6 having an SOI layer 7 with a thickness of
about 0.4 .mu.m was obtained.
Measurement of surface roughness:
First, as for surface roughness of a delaminated wafer as it is
obtained in FIG. 1(e), i.e., a wafer having an SOI layer not
subjected to the two-stage heat treatment according to the present
invention at all, P-V (Peak to Valley) value and RMS value of its
surface (delaminated plane) were measured by atomic force
microscopy for 1 .mu.m square and 10 .mu.m square. The P-V values
were 56.53 nm in average for 1 .mu.m square, and 56.63 nm in
average for 10 .mu.m square. The RMS (root mean square roughness)
values were 7.21 nm in average for 1 .mu.m square, and 5.50 nm in
average for 10 .mu.m square.
Subsequently, the wafer having an SOI layer obtained through the
aforementioned steps of (a) to (e) shown in FIG. 1 was subjected to
an RTA treatment in a temperature range of 1000.degree. C. to
1225.degree. C. in an atmosphere containing hydrogen, and then its
surface roughness in terms of P-V value and RMS value was measured
for 1 .mu.m square and 10 .mu.m square by atomic force
microscopy.
The results of the above measurement are shown in graphs of FIGS.
2-5.
FIG. 2 shows a graph representing relationship among the RTA
treatment temperature, the treatment time and the P-V value for 1
.mu.m square. This graph shows that the RTA treatment in a
temperature range of 1000.degree. C. to 1225.degree. C. for several
seconds to several tens of seconds greatly improved the short
period components (1 .mu.m) of surface roughness compared with the
untreated one, and provided P-V values comparable to that of a
mirror-polished wafer (PW). In FIG. 2, values corresponding to the
temperatures of 1000, 1100, 1200 and 1225.degree. C. are plotted
with squares, triangles, rhomboids and circles, respectively.
FIG. 3 shows a graph representing relationship among the RTA
treatment temperature, the treatment time and the P-V value for 10
.mu.m square. It can be seen that the long period components (10
.mu.m) of surface roughness were gradually improved as the
treatment time became longer, unlike the case of the aforementioned
short period components, and, to obtain a P-V value comparable to
that of a mirror-polished wafer (PW), although it depended on the
treatment temperature, a period of around several thousands of
seconds was required even when the treatment was performed at, for
example, 1225.degree. C.
FIGS. 4 and 5 show the measurement results for the short period
components (1 .mu.m) and the long period components (10 .mu.m) of
surface roughness in terms of RMS values, respectively, and they
show relationship among the RTA treatment temperature, the
treatment time and the RMS value.
From the results shown in the graph of FIG. 4, it can be seen that,
although the RMS value decreased by the RTA treatment at
1200.degree. C. as the treatment time became longer and the surface
roughness tended to be improved with time, the RMS value was
markedly improved to a level of a mirror-polished wafer (PW) by the
RTA treatment for several seconds or several tens of seconds at any
temperature. The relationship of the symbols used for the plotting
and the temperatures in FIG. 4 is similar to that in FIG. 2.
On the other hand, as for the long period components of surface
roughness, unlike the case of the aforementioned short period
components, they were gradually improved as the treatment time
became longer as evident from the graph of FIG. 5, and, to obtain
an RMS value comparable to that of a mirror-polished wafer (PW),
although it depended on the treatment temperature, it can be seen
that a period of around several thousands of seconds was required
even when the treatment was performed at, for example, 1225.degree.
C.
From the above results, it can be seen that the short period
components (about 1 .mu.m) of surface roughness are greatly
improved to a level comparable to that of a mirror-polished wafer
by the RTA treatment for an extremely short period of time (several
seconds or several tens of seconds), whereas the long period
components (about 10 .mu.m) cannot be made to be at a level
comparable to that of a mirror-polished wafer, although it depends
on the treatment temperature, unless the heat treatment is
performed for a long period of time (several thousands of seconds
or more).
EXAMPLES 1 AND 2 AND COMPARATIVE EXAMPLE 1
Heat treatment of SOI wafers:
An SOI wafer produced under the same condition as the SOI wafer
used for the aforementioned heat treatment test using the RTA
apparatus according to the steps (a) to (e) shown in FIG. 1 was
subjected to a heat treatment by an RTA apparatus under each of the
conditions shown in Table 1 (100% hydrogen atmosphere), and then
subjected to a heat treatment by a batch processing type furnace
(100% argon atmosphere) to obtain an SOI wafer, which was subjected
to the two-stage heat treatment according to the present invention
(Examples 1 and 2). Separately, there was also prepared a wafer
subjected to the heat treatment by the RTA apparatus but not
subjected to the heat treatment by the batch processing type
furnace thereafter
COMPARATIVE EXAMPLE 1
TABLE 1 Heat Heat treatment by treatment Heat treatment by batch
processing condition RTA apparatus type furnace Example 1
1225.degree. C., 10 seconds 1200.degree. C., 1 hour Example 2
1200.degree. C., 30 seconds 1200.degree. C., 1 hour Comparative
1225.degree. C., 10 seconds None Example 1
Surface roughness measurement:
Surface roughness (RMS value) of the SOI wafers obtained in the
aforementioned Examples 1 and 2 and Comparative Example 1 was
measured before and after the heat treatment by atomic force
microscopy for 1 .mu.m square and 10 .mu.m square, and the results
are shown in Table 2.
TABLE 2 Before heat After heat Results of treatment treatment
surface (RMS: nm) (RMS: nm) roughness 1 .mu.m 10 .mu.m 1 .mu.m 10
.mu.m measurement square square square square Example 1 7.21 5.50
0.18 0.28 Example 2 7.50 5.80 0.20 0.30 Comparative 7.45 5.75 0.21
1.60 Example 1
As clearly seen from the results shown in Table 2, substantially no
difference of the RMS values before the heat treatment was seen
among the wafers for both of 1 .mu.m square and 10 .mu.m
square.
On the other hand, after the heat treatment, although there was
almost no difference of the values among the wafers for 1 .mu.m
square, the values for 10 .mu.m square of the wafers of Examples 1
and 2 were greatly improved to a level near their RMS values for 1
.mu.m square. However, as for the wafer of Comparative Example 1,
which was not subjected to the heat treatment by the batch
processing type furnace, the RMS value for 1 .mu.m square was
greatly improved, whereas the RMS value for 10 .mu.m square was
significantly larger than those of the wafers of Examples 1 and 2,
and thus it can be seen that the long period components of surface
roughness were not improved sufficiently.
EXAMPLES 3 AND 4
Production of bond wafers:
A silicon single crystal ingot was produced by the CZ method with
applying a magnetic field, in which the pulling condition (V/G) was
controlled to reduce grown-in defects in the crystal. This ingot
was processed in a conventional manner to produce a mirror-surface
CZ wafer (diameter: 200 mm, crystal orientation <100>) of
which COPs were reduced for the whole crystal (Example 3). COPs and
haze level on the surface of this wafer were measured by using a
surface inspection apparatus (SP-1, produced by KLA/Tencor Co.,
Ltd.), and it was found that no COP having a diameter of 0.12 .mu.m
or more existed on the surface and the haze level of the mirror
surface was about 0.03 ppm in average.
Separately, a CZ mirror-surface wafer (diameter: 200 mm, crystal
orientation <100>), which was produced from a silicon single
crystal ingot pulled with usual pulling condition (pulling rate:
1.2 mm/min), was loaded into an epitaxial growth apparatus to
produce an epitaxial wafer having an epitaxial layer with a
thickness of 10 .mu.m at 1125.degree. C. (Example 4).
The number of COPs having a diameter of 0.12 .mu.m or more existing
on the CZ wafer surface before the deposition of the epitaxial
layer was about 1000 per wafer in average. Haze level of the
epitaxial layer surface was about 0.2 ppm in average. There also
was a wafer having projections called mounds.
Production of SOI wafers:
Ten wafers for each of the wafers produced as bond wafers by the
methods of the aforementioned Example 3 and Example 4 were
prepared, and SOI wafers were produced under the same conditions as
the aforementioned heat treatment test by the RTA apparatus
according to the steps (a) to (e) shown in FIG. 1. By inspecting
the SOI surfaces and bonding surfaces after the delamination,
existence of voids or blisters was investigated and cause of their
generation was examined. As a result, it was confirmed that no void
considered to be generated due to the haze or protrusions of the
bond wafer surfaces (bonding surfaces) was observed at all for the
SOI wafers produced by using the wafers of Example 3, whereas voids
or blisters considered to be generated due to haze or mounds on the
epitaxial layer surfaces were present on three SOI wafers of ten
SOI wafers produced by using the wafers of Example 4.
Heat treatment of SOI wafers:
The aforementioned SOI wafers of Example 3 and Example 4 after the
delamination were subjected to a heat treatment at 1225.degree. C.
for 3 hours by using a batch processing type furnace in an
atmosphere of 97% argon/3% hydrogen, without polishing the SOI
layer surfaces.
Surface roughness measurement:
The surface roughness of the aforementioned SOI wafers obtained in
Examples 3 and 4 was measured for 1 .mu.m square and 10 .mu.m
square before and after the heat treatment. The results are shown
in Table 3.
TABLE 3 Before heat After heat treatment (RMS: nm) treatment (RMS:
nm) 1 .mu.m 10 .mu.m 1 .mu.m 10 .mu.m square square square square
Example 3 7.33 5.60 0.18 0.33 Example 4 7.42 5.73 0.19 0.35
While the present invention was explained above with reference to
the examples, the present invention is not limited to the
embodiments described above. The above-described embodiments are
mere examples, and those having the substantially same structure as
that described in the appended claims and providing the similar
functions and advantages are included in the scope of the present
invention.
For example, while CZ wafers and epitaxial wafers were used in the
aforementioned examples, wafers that can be used in the present
invention are not limited to those, and FZ wafers and hydrogen
(argon) annealed wafers can also be used.
Further, while the present invention was explained above mainly for
the cases of bonding two of semiconductor wafers (silicon wafers),
the present invention is not limited to those, and can similarly be
applied to cases where a semiconductor wafer and an insulating
substrate (for example, substrates of quartz, sapphire, alumina
etc.) are directly bonded to produce SOI wafers.
* * * * *